Currently the liquid crystal compounds are widely applied in various types of displays, electro-optical devices, sensors and the like. Among the large number of liquid crystal compounds used in display area, the nematic liquid crystal is most widely used. Nematic liquid crystals have been used in passive TN, STN matrix display and TFT active matrix system.
Although the technology of thin-film-transistor liquid-crystal display (TFT-LCD) has been mature, it should be continuously improved to decrease the response time, to widen the phase transition temperature, to decrease the driving voltage, etc. It has been well-known that liquid crystals play an important role to improve the characteristics of LCDs. The liquid crystal compounds are required to have high chemical and thermal stability, as well as high stability in electric field and electromagnetic radiation. The liquid crystal compounds used in TFT-LCD not only should be stable to heat, UV light, electric field, and electromagnetic radiation, but also have the wide temperature range of a nematic phase, appropriate optical anisotropy, very high resistance, high voltage holding ratio, and low vapor pressure.
For dynamic picture display such as LCD-TV, in order to improve the characteristics of the device to reduce the picture motion blur and trailing, it is desired that the liquid crystal mixtures have appropriate physical properties with a short response time, a low viscosity (γ1), and a low driving voltage. Also it is great significance to elevate the dielectric anisotropy (Δ∈) of the liquid crystal compound for the modulation of liquid crystal mixture.
A large number of studies have shown that a introduction of difluoroethylene methoxy group (—CF2O—) into a liquid crystal molecules will reduce rotational viscosity γ1. In addition, due to the contribution of dipole moment of difluoromethyleneoxy bridge (—CF2O—), the dipole moment of the terminal fluorine group also increases to some extent, so that the dielectric anisotropy Δ∈ of the liquid crystal molecules increases. Merck of Germany and Chisso of Japan have reported several liquid crystal compounds containing various substituent and difluoromethyleneoxy group (—CF2O—) (CN1717468A, CN101143808A, CN101157862A). However the introduction of difluoromethyleneoxy bridge (—CF2O—) will reduce clearing point of the liquid crystal. When the liquid crystal mixture is modulated, monomer having higher melting point and higher viscosity is needed for balancing the reduction of clearing point. thus limiting the improvement of response speed of the liquid crystal mixture.